A. Field of the Invention
The present invention relates to a magnetic domain wall motion-detecting terminal-possessing magnetic recording medium, and more particularly relates to a magnetic domain wall motion-detecting terminal-possessing magnetic recording element which carries out data storage by changing the state of magnetization of a ferromagnetic body.
B. Description of the Related Art
Currently, volatile memories and nonvolatile memories are used in personal computers and peripherals. With a volatile memory such as a DRAM, held data is lost when the power supply is turned off, but the rewriting and reading speeds and the degree of integration are excellent. On the other hand, with a nonvolatile memory such as a flash memory, the writing and reading speeds are poorer, but there is the advantage that data continues to be held even when the power supply is turned off.
An ideal form of memory would be one that has the advantages of both a volatile memory and a nonvolatile memory, i.e., one that has fast writing and reading speeds, and yet that continues to hold data even when the power supply is turned off. One of the most promising of such devised next generation memories is a magnetic random access memory (MRAM).
A magnetic random access memory (MRAM) according to the prior art shown in FIGS. 5A and 5B is a memory that uses a magnetic tunnel junction (MTJ) element 11 having as a basic structure a multi-layer structure of ferromagnetic free layer 12, insulator layer 13, and ferromagnetic fixed layer 14. With such an MTJ element 11, binary data is produced using the property that the resistance of a tunnel current flowing in the direction through the layers differs according to whether the magnetization directions of ferromagnetic free layer 12 and ferromagnetic fixed layer 14 are parallel or anti-parallel. The magnetization direction of a ferromagnetic body is maintained even when the current is turned off, and hence such an MRAM is a nonvolatile memory. In the drawings, the arrows within the layer indicate magnetization directions of the layers. The layer with bi-directional arrow indicates the layer in which magnetic inversion takes place.
As the structure of an MRAM, a structure in which MTJ elements 11 are disposed at points of intersection between bit lines 15 and write word lines 16 wired in a perpendicular matrix shape as shown in FIG. 5B is generally adopted. Each MTJ element 11, together with a MOS transistor that acts as a switching element for cell selection, constitutes a 1 bit cell.
Writing is carried out by passing a current through both bit line 15 and write word line 16. Upon the current being passed through both bit line 15 and write word line 16, a magnetic field induced from each of these wires is applied to the point of intersection, whereby the magnetization of ferromagnetic free layer 12 can be reversed. With bit line 15 or write word line 16 alone, the switching magnetic field required for reversing the magnetization of ferromagnetic free layer 12 is not obtained, and hence writing can be carried out to only the bit cell at the point of intersection between bit line 15 and write word line 16. Writing can thus be carried out to any chosen bit cell.
Reading is carried out by selecting a desired bit line 15 and read word line 17, and measuring the resistance of a current flowing between the bit line 15 and a reading electrode 18 connected to the read word line 17. The resistance of an MTJ element 11 can take either of two values depending on the combination of the magnetization directions of ferromagnetic free layer 12 and ferromagnetic fixed layer 14, and hence by setting an intermediate value therebetween as a reference voltage, either of two data values “1” and “0” can be obtained depending on the measured resistance.
Furthermore, in recent years, there has been developed a spin injection magnetization reversal MTJ element in which the magnetization of ferromagnetic free layer 12 is reversed by passing a spin polarized current instead of applying a magnetic field due to a current flowing through each of bit line 15 and write word line 16, and an MRAM using such spin injection magnetization reversal MTJ elements.
FIG. 6 is a view showing the structure of an MRAM using a spin injection magnetization reversal technique proposed in Japanese Patent Application Laid-open No. 11-120758. Writing is carried out as follows. Consider a current being passed such that electrons are injected from ferromagnetic fixed layer 14 into ferromagnetic free layer 12. The spin of an electron passing through ferromagnetic fixed layer 14 undergoes exchange interaction with the magnetization of ferromagnetic fixed layer 14 and thus receives spin torque from the magnetization, and hence is polarized in the magnetization direction of ferromagnetic fixed layer 14. When the spin polarized electron enters ferromagnetic free layer 12, the electron now gives spin torque to the magnetization of ferromagnetic free layer 12. In this way, the magnetization of ferromagnetic free layer 12 is aligned so that it is parallel with the magnetization of ferromagnetic fixed layer 14.
On the other hand, when a current is passed such that electrons are injected from ferromagnetic free layer 12 into ferromagnetic fixed layer 14, electrons each having a spin anti-parallel to the magnetization of ferromagnetic fixed layer 14 are reflected at the interface between ferromagnetic fixed layer 14 and insulator layer 13, and the reflected electrons give a spin torque to the magnetization of ferromagnetic free layer 12. As a result, the magnetization of ferromagnetic free layer 12 becomes anti-parallel to the magnetization of ferromagnetic fixed layer 14.
Through the above effects, by selecting the direction of the current applied to the multi-layer film, the magnetizations of ferromagnetic fixed layer 14 and ferromagnetic free layer 12 can be made to be parallel or anti-parallel to one another. Actually carrying out writing by reversing the magnetization of ferromagnetic free layer 12 using a current requires a current greater than a certain current, i.e., a critical current. When reading, a current less than the critical current is passed, and the data is read by measuring the resistance as with a conventional MRAM.
With an MRAM using the spin injection magnetization reversal technique, compared with a conventional MRAM, the write word lines 16 for producing a writing magnetic field are not required, and hence there is an advantage that the structure of the element can be simplified. However, with the spin injection magnetization reversal technique, the critical current density required for magnetization reversal is approximately 5×107 A/cm2, and hence there is a problem that the current density is high.
In Japanese Patent Application Laid-open No. 2005-191032, there is thus proposed an MRAM of a type in which a magnetic domain wall in ferromagnetic free layer 12 is moved using a current-driven magnetic domain wall motion technique of moving the magnetic domain wall in the ferromagnetic body by applying a current, instead of the spin injection magnetization reversal technique. It is thought that current-driven magnetic domain wall motion is produced through two effects, namely magnetization alignment due to spin torque given to the magnetization of the ferromagnetic body by the electron spin of the applied current, and momentum transferring from the electrons to the magnetic domain wall accompanying electron scattering by the magnetic domain wall.
Following is a description of the MRAM using the current-driven magnetic domain wall motion technique proposed in Japanese Patent Application Laid-open No. 2005-191032 with reference to FIG. 7. Insulator layer 13 and ferromagnetic fixed layer 14 are laminated on ferromagnetic free layer 12, and a read word line (not shown) is connected to ferromagnetic fixed layer 14 via reading electrode 18. On the other hand, writing electrodes 19a and 19b are formed at respective ends of ferromagnetic free layer 12.
As shown in FIG. 7A, when magnetic domain wall 20 is to the left in the drawing of a multi-layer portion including ferromagnetic fixed layer 14, and the magnetizations of ferromagnetic free layer 12 and ferromagnetic fixed layer 14 are aligned parallel to one another, if a current is passed to writing electrode 19b from reading electrode 18, then the element exhibits low resistance.
To carry out data recording, current 21 is passed from writing electrode 19b to writing electrode 19a. Through the application of the current, magnetic domain wall 20 moves to the right in the drawing, and hence the magnetization of ferromagnetic free layer 12 at the multi-layer portion and the magnetization of ferromagnetic fixed layer 14 become anti-parallel to one another. If a current is passed from reading electrode 18 to writing electrode 19b in this state, then the element now exhibits high resistance.
As described above, using the current-driven magnetic domain wall motion technique, the magnetization of ferromagnetic free layer 12 of the MTJ element can be reversed and yet, unlike with an MRAM using the spin injection magnetization reversal technique, current is applied to only the ferromagnetic free layer, and hence there is the advantage that the power consumption can be reduced.
However, with MRAMs of the prior art, it has been difficult to achieve both making the elements minute so as to increase the recording density and simplifying the structure to realize this, and reducing the writing current. With the MRAM using the spin injection magnetization reversal technique, simplification of the structure is realized by omitting the writing elements. However, the current density required for the spin injection magnetization reversal has not yet been reduced to an extent that practical implementation is possible. On the other hand, with the MRAM using the current-driven magnetic domain wall motion technique, electrodes for applying a writing current for moving the magnetic domain wall in the ferromagnetic free layer are formed, whereby the current required for writing is reduced. However, it is still necessary to form a multi-layer portion including ferromagnetic fixed layer 14, and hence the structure is complex.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.